Acute kidney injury (AKI) is a devastating complication of critical illness. No pharmacologic therapies reliably prevent or treat AKI. Novel therapeutic targets for clinical intervention are therefore needed. Vitamin D metabolites have important and potentially beneficial antiinflammatory effects which prevent AKI in animal models. Since vitamin D metabolites are often decreased in critical illness, we propose that deficiencies in the vitamin D axis may be risk factors for incident AKI, and that exogenous vitamin D metabolites may be novel therapeutic agents for AKI prevention. Our preliminary data support these hypotheses. We showed that patients with established AKI have decreased levels of 25-hydroxyvitamin D (25D) and 1,25-dihydroxyvitamin D (1,25D) in association with elevated fibroblast growth factor 23 (FGF23) levels; the latter is known to suppress both renal and extra-renal conversion of 25D to 1,25D. In further preliminary data, we recently published the results of a pilot RCT demonstrating that administration of a single 2 g IV dose of 1,25D to critically il patients increases leukocyte mRNA expression of interleukin-10 (IL-10) and heme oxygenase-1 (HO-1). These important antiinflammatory proteins prevent AKI in animal models. In Aim 1 of this proposal, we will leverage an established ICU cohort to perform the first comprehensive, prospective study to investigate whether 25D, 1,25D, and FGF23 are independent risk factors for adverse renal outcomes among 200 critically ill patients with a high AKI event rate. Primary outcomes will include: incident AKI, assessed by serum creatinine; renal tubular injury, assessed by urinary NGAL and KIM- 1; and need for renal replacement therapy or death. In Aim 2 we will enroll 150 critically ill patients at risk of AKI into a three-arm, double-blind, randomied study of 25D, 1,25D, or placebo to test the effects of repeated doses of vitamin D metabolites on renal and immunomodulatory endpoints. We will test whether 25D and/or 1,25D attenuate incident kidney injury, assessed by time averaged levels of both serum creatinine and urinary NGAL and KIM-1. Further, we will test whether 25D and/or 1,25D increase monocyte mRNA expression of antiinflammatory targets IL-10, HO-1, HIF1a, and thrombomodulin, decrease monocyte mRNA expression of proinflammatory targets ICAM-1 and IL-6, and increase T regulatory (Treg) %, all of which attenuate kidney injury in animal models. Additionally, we will evaluate physiologic endpoints including plasma levels of FGF23 and monocyte mRNA expression of 1-a hydroxylase and 24-hydroxylase to elucidate whether peripheral conversion of 25D to 1,25D is impaired in critical illness. By unifying a comprehensive, prospective clinical outcomes study and an innovative trial under one thematic umbrella, we will generate novel, clinically relevant insights into the emerging role of vitamin D metabolites in critical illness an AKI that could have important diagnostic and therapeutic implications.